This invention relates to the acid hydrolysis of aqueous slurries of carbohydrates such as starches derived from cereal grains, amylaceous roots and tubers, and cellulose in the form of saw dust, wood chips, bark, paper, rags, etc.
With the ever-increasing depletion of economically recoverable petroleum reserves, the production of ethanol from vegetative sources as a partial or complete replacement for conventional fossil-based liquid fuels becomes more attractive. In some areas, the economic and technical feasibility of using a 90% unleaded gasoline-10% anhydrous ethanol blend ("gasohol") has shown encouraging results. According to a recent study, gasohol powered automobiles have averaged a 5% reduction in fuel compared to unleaded gasoline powered vehicles and have emitted one-third less carbon monoxide than the latter. In addition to offering promise as a practical and efficent fuel, biomass-derived ethanol in large quantities and at a competitive price has the potential in some areas for replacing certain petroleum-based chemical feedstocks. Thus, for example, ethanol can be catalytically dehydrated to ethylene, one of the most important of all chemical raw materials both in terms of quantity consumed and versatility in product synthesis.
The various operations in processes for obtaining ethanol from such recurring sources as cellulose, cane sugar, amylaceous grains and tubers, e.g., the separation of starch granules from non-carbohydrate plant matter and other extraneous substances, the chemical and/or enzymatic hydrolysis of starch to fermentable sugar (liquefaction and saccharification), the fermentation of sugar to a dilute solution of ethanol ("beer") and the recovery of anhydrous ethanol by distillation, have been modified in numerous ways to achieve improvements in product yield, production rates and so forth (see, for example, U.S. Pat. No. 3,236,740 and the booklet "Industrial Alcohol by Continous Fermentation and Vacuum Distillation With Low Energy Consumption", of Chemapec, Inc., Woodbury, New York). For ethanol to realize its vast potential as a partial or total substitute for petroleum fuels or as a substitute chemical feedstock, it is necessary that the manufacturing process be as efficient in the use of energy and raw materials as possible so as to maximize the energy return for the amount of ethanol produced and enhance the standing of the ethanol as an economically viable replacement for petroleum based raw materials. To date, however, relatively little concern has been given to the energy and raw material requirements for manufacturing ethanol from biomass and consequently, little effort has been made to minimize the thermal expenditure and waste incurred in carrying out any of the aforesaid discrete operations involved in the manufacture of ethanol from vegetative sources.
Processes for the acid hydrolysis of carbohydrate polymers i.e., starch and cellulose, to provide fermentable sugars are known (viz., U.S. Pat. Nos. 2,203,325; 2,210,659; 2,359,763; 2,393,095; 2,395,907; 2,529,131; 2,565,404; 2,946,706; 2,954,304; 2,989,425; 3,169,083; 3,200,012; 3,236,687; 3,313,654; 3,446,664; 3,484,287; 3,607,395; 4,137,094; and, 4,155,884).
In addition to the desired reaction whereby the carbohydrate polymer molecules are split into fermentable sugars, other reactions taking place during hydrolysis tend to reduce the maximum theoretical conversion of available carbohydrate to such sugars and produce nonfermentable hydrolysate products. Three of the principle types of undesirable reactions known to take place in acid catalyzed carbohydrate polymer hydrolysis are: degradation (starch molecule is irreversibly destroyed to provide 5-hydroxymethylfurfural which hydrolyzes to levulinic acid and formic acid, and separately polymerizes to humins); reversion (glucose repolymerizes and/or isomerizes to unfermentable substances); and retrogradation (hydrolysis splits out the branched chain components of the starch molecule leaving a straight chain, lower molecular weight water insoluble polymeric molecule which crystallizes at about 70.degree.-80.degree. C. and becomes resistant to further hydrolysis). In a typical acid hydrolysis process, when equilibrium has been achieved, from about 15 to about 20 weight percent of the depolymerized starch will be present in the form of one or more of the foregoing non-fermentable hydrolysates, the balance of the depolymerized starch being present as glucose and/or other fermentable sugar(s). To the extent nonfermentable products are produced side-by-side with fermentable sugar(s), they represent a loss in yield of the hydrolysis reaction and compromise the usefulness of acid hydrolysis as a process for obtaining fermentable sugar on a large-scale, economical basis.
According to U.S. Pat. No. 2,529,131 referred to supra, the non-fermentable hydrolysate products resulting from one or more of the aforesaid undesirable reactions eventually is recovered in the stillage, or "vinasse", obtained as a result of the distillation of the dilute ethanol, or "beer", resulting from the fermentation of the fermentable sugar portion of the hydrolysate. To maximize overall ethanol production, based on the original quantity of carbohydrate polymer employed, it is proposed in U.S. Pat. No. 2,529,131 to subject the stillage to further acid hydrolysis to convert the unfermentable products therein to fermentable sugars.